1. Field of the Invention
The present invention relates to a solid state imaging device, e.g., an infrared solid state imaging device or the like including a semiconductor substrate and a plurality of photosensitive cells formed by depositing on the semiconductor substrate a conductive material which is different from the substrate, and more particularly the invention relates to such solid state imaging device in which each of the cells further includes a reflecting film for reintroducing the passed light into its photosensing portion, and a method for producing the same.
2. Description of the Prior Art
An infrared solid state imaging device is an example of solid state imaging devices of the type including a plurality of photosensitive cells formed on a semiconductor substrate by depositing a material different from the substrate on it, each of said cells being provided with a reflecting film for reintroducing the passed light into its photosensing portion.
As is well known in the art, this type of solid state imaging device must satisfy a requirement that "the incident light on each of the picture elements or the photosensitive cells must be photoelectrically converted by the picture element alone". If the light incident on one picture element is photoelectrically converted by another picture element (this phenomenon is referred to as a "crosstalk"), there is the disadvantage that an output image is different from the original image formed on the solid state imaging device and the solid state imaging device fails to attain its essential purposes.
A disadvantage of the conventional solid state imaging device resides in that the previously mentioned crosstalk cannot be prevented fully and an excellent image cannot be produced.
In detail, according to the conventional production process of the solid state imaging device, signal charge transfer units of the CCD type or the like are formed firstly on the surface of a semiconductor substrate and metal electrodes for driving the units are formed on the units, and then the photosensitive cells serving as picture elements are formed on the substrate. Thereafter, the electrodes and the cells are covered by an insulating film layer, and the surface of the insulating film layer is flattened to form the previously mentioned metal reflecting films on the portions of the flattened surface of the insulating film corresponding to the photosensitive cells.
In this case, the insulating film layer is composed of an oxide film formed for example by the CVD (chemical vapor deposition) process to a thickness of about 4,000 to 10,000 angstrom and its surface shape is flattened when subjected to a heat treatment at a temperature of about 900.degree. C. The flattening by such heat treatment is widely known as a reflow treatment and this is effective in the prevention of electrical faults (breaking, short-circuiting, etc.,) of aluminum wiring formed on the oxide film by the following operation.
On the other hand, while each metal reflecting film should preferably include a reflecting surface of as flat as possible in parallel to the photosensitive cell, after the insulating film layer has been subjected to the flattening treatment, the surface shape of the recess of each photosensitive cell portion formes a curved surface, particularly at the marginal portions of the photosensitive cell and therefore the reflecting surface of the metal reflecting film formed on the photosensitive cell is correspondingly formed into a curved surface at the marginal portions of the reflecting surface.
The reflecting surface having such a curved surface cannot return all of its incident light to the original optical path so that a part of the light reflected by the curved surface is directed to the photoelectric conversion portion of the adjoining different photosensitive cell and this results in the generation of a pseudo signal due to the crosstalk of the light between the adjoining picture elements, thereby causing a bleeding of the resulting image.
Another problem of the conventional solid state imaging devices is derived from the fact that particularly in recent years the various solid state imaging devices including the infrared solid state imaging device are required from all quarters to fulfil such performance that the chip size is reduced further, the spatial resolution is increased further (hence the number of picture elements is increased further) and the photosensitivity is enhanced further at the same time.
In order to satisfy these requirements, it is necessary to increase the area ratio (opening ratio) occupied by a photosensitive cell within each unit picture element of the solid state imaging device, and various attempts have been made for this purpose. Decreasing the width of an isolation area composed of an oxide insulating layer formed for example by an LOCOS (localized oxidation of silicon) process to electrically isolate each photosensitive cell from the surroundings within a semiconductor substrate and decreasing the width of BCCD diffused layers formed adjacent to the photosensitive cells for charge transfer purposes may be cited as examples of such attempts. However, each of such attempts alone cannot fully increase the opening ratio of the photosensitive cells and thus it is difficult to obtain an imaging device having a high photosensitivity.
Then, while it is desirable to form by etching a wide opening of a sufficient area in the insulating film during the formation of the photoelectric conversion portion of the photosensitive cell, the conventional manufacturing methods of solid state imaging devices inevitably cause the occurrence of side etching in the case of relatively thick insulating films (of about 4,000 to 10,000 angstrom) so that if the opening formed in the resist layer for the purpose of etching is so large, the side etching of the insulating film results in the exposure of the CCD electrode adjoining the photosensitive cell and thus there is a limitation to increasing the opening ratio of the photosensitive cells from the structural point of view.
Another disadvantage of the conventional solid state imaging device is the danger of the plurality of photosensitive cells becoming nonuniform in characteristic and more particularly there have been the cases where the photosensitive cells are changed in characteristic during the cleaning of the imaging device, its assembling to a camera and its replacement.
In accordance with the studies made by the inventors, such nonuniformity and variation in the photosensitive characteristic of the plurality of photosensitive cells are caused by the fact that there is caused the electrification by static electricity or a negative charge of the metal reflecting film facing through the insulating film the photoelectric conversion portion composed of a semiconductor Schottky junction or the like in each photosensitive cell and the dark current in the photoelectric conversion portion is varied to increase or decrease by the charged potential, thereby causing the increased dark current to add a pseudo signal to the essential signal charge photoelectrically converted by the photosensitive cell.
For instance, during the manufacture of an imaging device a silicon wafer serving as a semiconductor substrate is cleaned by the high pressure spraying of pure water (the specific resistance is 18 megohm or over) from which various ions have been removed. When a jet stream of cleaning water having such a high specific resistance is sprayed against the wafer, static electricity is generated and therefore the metal reflecting films formed on the wafer through the insulating film are electrified with negative charges.